Method of separating gaseous mixtures



July 26, 1960 c. J. SCHILLING V 2,946,200

METHOD OF SEPARATING GASEOUS MIXTURES Filed June 9, 1955 INVENTOROLARENGE'J. saw/ave ATTORNEY ,fractionating operations.

Unite States Patent 2,946,200 METHOD OF EPARATING GASEOUS MIXTURESClarence J. Schilling, Allentown, Pa, assignor to Air ProductsIncorporated, a corporation of Michigan- Filed June 9, 1955, Ser. No.514,213

7 Claims. (Cl. 62-42) This invention relates to improvements in theseparation of mixtures of component gases having different boilingpoints by low temperature fractionation and more particularly to thegeneration of liquid reflux for a fractionating operation.

In the separation of mixtures of component gases into a liquid highboiling point fraction and a gaseous low boiling point fraction by afractionating operation employing a column having a liquid-vapor contactzone, liquid reflux or wash liquid for the column is frequently obtainedby liquefying gaseous fraction originating in and flowing upwardly fromthe liquid-vapor contact zone of the column. This is generallyaccomplished by means of a condenser structure mounted at the top of thecolumn. Condenser structures provided for this purpose usually include aplurality of spaced vertically disposed elongated tubular members ofrelatively small diameter positioned with their lower ends discharginginto the liquid-vapor contact zone of the column and with their upperends communicating with a closedchamber formed by the dome of thecondenser'structure. -A separate passageway in the form of a conduit ofrelatively large diameter is provided for conducting gaseous fractionfrom the column into the closed chamber. The tubular members areenclosed in a chamber containing a relatively cold fluid at atemperature below the boiling point of the gaseous fraction at theexisting pressure to effect liquefaction of the gaseous fraction in thetubular members. Liquefied gaseous fraction flows downwardly in thetubular members, due to the action of gravity, and is discharged fromtheir lower ends into the column. This action produces a suction effectdrawing gaseous fraction from the closed chamber downwardly into thetubular members for liquefaction by heat exchange with the relativelycold fluid.

Gaseous mixtures usually include component gas or gases having such lowboiling points as not to be liquified at the pressure and :temperatureexisting during normal Such gases are known as incondensables orincondesable gases. The incon densable gases, having relatively lowboiling points, are

mixed with the gaseous low boiling point fraction and flow upwardlytherewith into the condenser and collect in the closed chamber formed bythe condenser dome.

,Such accumulation of incondensables reduces the condenser action anddecreases fractionating efficiency due to an insuflicient supplyofreflux liquid. For example, in the separation of air into liquidoxygen and gaseous nitrogen products, the incondensable gases presentcomprise hydrogen, neon and helium which flow with the gaseous nitrogenfraction into the condenser structure. Although these incondensablegases constitute less than one-tenth of one percent of the air feedmixture, unless they are removed from the condenser structurethe supplyof'liquid reflux will be materially impaired. This results from the factthat the incondensable hydrogen, neon and helium gases accumulating inthe condenser tubes lower the-partial pressure of the' gaseous nitrogenfraction 27 therein so that the existing pressure and temperature areinsutficient to condense the gaseous nitrogen fraction at the requiredrate, and since the incondensable gases become concentrated in thecondenser tubes upon condensation of the gaseous nitrogen fraction. Inorder to overcome these difliculties it has been the general practice tobleed the dome of the condenser structure and thus remove a suflicientquantity of the incondensable gases for adequate operation.

It has been discovered that the provision of means for bleeding the domeof the condenser structure does not overcome the problems presented byincondensable gases when the incondensable gases constitute more thanone percent of the feed mixture. While it cannot be definitely statedwhy incondensable gases in excess of one percent of the feed mixture arenot sufliciently bled from the condenser dome to prevent a reduction inthe production of reflux liquid, it has been determined that aproportion of the incondensable gases are drawn into the condenser tubesby the suction or pumping action of the condensing gaseous low boilingpoint fraction and a decrease in the liquefaction of the gaseous lowboiling point fraction results for reasons discussed above. It has alsobeen determined that the provision of means for bleeding the condenserdome is inadequate to overcome the problem presented by incondensablegases in excess of one percent of the feed mixture even whenthecondenser dome is bled in such a manner as to continuously withdraw alarge proportion of the gaseous low boiling point fraction.

The present invention overcomes the foregoing problem by providing anovel method of producing liquid reflux for a fractionating operation bywhich any desired proportion ofcondensable gaseous low boiling pointfraction may be continuously liquefied to provide a constant supply ofliquid .reflux for the fractionating operation irrespective of thepresence of incondensable gases in the gaseous low boiling pointfraction in excess of one percent of the feed mixture. According to thedisclosed method the gaseous fraction of the fractionating opera tion,comprising a condensable gaseous low boiling point component andincondensable gas or gases of still lower boiling point, is circulateddownwardly through the condenser tubes by a force, in addition to thepumping or suction'eflect produced by the action of gravity on thecondensed gaseous low boiling point fraction, to prevent theaccumulation of incondensable gases in the condenser tubes and aconcomitant drop incondenser efiiciency.

This circulating force is established by withdrawing a stream of gaseousfraction from a zone located below the condenser tubes in communicationwith the lower ends of the condenser tubes and isolated from thefractionating zone with respect to vapor. The withdrawn stream ofgaseous mixture includes the incondensable gases and may comprise aportion of the condensable gaseous low boiling point component of thegaseous fraction. i

The foregoing will be more fully understood with reference to thefollowing detailed description considered in connection with theaccompanying drawing which discloses a fraetionating column forproducing pure argon which is designed to operate according to theprinciples of the present invention. It is to be expressly understoodhowever that the novel method provided by the present invention is notrestricted to the environment of argon production but has broadapplication to the fractionation of gaseous mixturesincludingincondensablegases exceeding one percent of the mixture, suchas in theseparation of hydrocarbons by low temperature fractionation forexample, and the disclosed environment is not intended to define thelimits of the invention, reference for the latter purpose being hadtothe appended claims.

The single figure of the drawing is a diagrammatic presentation of afractionating apparatus for producing a substantially pure argondesigned in such a manner as to operate in accordance with theprinciples of the present invention. I a

In a patent application of George Fedorko, Serial Number 509,449, filedMay 19, 1955, now Patent No. 2,909,410, for Argon Recovery, there isdisclosed a sys tem for obtaining substantially pure argon in which anargon fraction containing oxygen is obtained from a side column fed troma conventional two stage air fractionating column and in which the argonfraction containing oxygen is mixed with ammonia in critical proportionsto produce a reaction product comprising a mixture of argon, nitrogen,hydrogen and water vapor. The reaction product may comprise a mixtureincluding from 10% to 25% nitrogen, from 1% to 5% hydrogen and thebalance argon, depending upon the oxygen concentration in the argonfraction and upon other operating conditions. The reaction mixture afterdrying is'fractionated, such as in a fractionation column 10, to obtaina substantially pure argon product. The reaction mixture is introducedby way of a conduit 11 to the mid point of the fractionating columnwhich presents a fractionating zone provided with a stack of verticallyspaced iractionating trays 12 of conventional construction. In thecolumn 10 the reaction product is separated into a liquid high boilingpoint fraction comprising substantially pure argon which collects in apool 13 in the base of the column, and a gaseous fraction comprising thelow boiling point components, nitrogen and hydrogen, of the reactionproduct stream as well as a proportion of the argon which flows upwardlyin the column.

Liquid reflux for the column 10 is obtained by liquefying a portion ofthe nitrogen components of the gaseous fraction in a condenser structure14 provided at the upper end of the column. The condenser structureincludes a centrally positioned tube 15 of relatively large diameter anda plurality of relatively small diameter tubes 16 positioned in spacedrelation about the tube 15. The tubes 15 and 16 are vertically disposedwith their upper ends communicating with a chamber 17 formed by a closeddome 18 of the condenser structure. The lower end of the tube 15communicates with the fractionating zone of the column, while the lowerends of the small diameter tubes 16 open into an annular chamber 19formed by a horizontally disposed plate member 20 extending between theouter surface of the tubular member 15 and the inner walls of the columnin a'plane spaced abstracted from the nitrogen causes the liquid argonto boil and provide a vapor source for column operation. From theboiling coil 30, the nitrogen stream is conducted through a conduit 31to an expansion valve 32 by which its pressure is reduced with a furtherdecrease in temperature to a sufiiciently low level to effectliquefaction of the nitrogen component in the tubes 16; from theexpansion valve the nitrogen stream is conducted by way of a conduit 33to the inlet conduit 28 of the chamber 21.

Liquid argon product may be withdrawn from the column through a conduit34.

The gaseous fraction produced upon fractionation of the reactionmixture, comprising a mixture of condensable argon and nitrogen, andincondensable hydrogen, flows upwardly through the central tube 15 intothe chamber 17, and from the chamber 17 downwardly into the tubes 16 inheat exchange relation with the relatively colder fluid circulatingthrough the chamber 21. Since the fluid in the chamber 17 is below theboiling point of nitrogen at the existing pressure, nitrogen and argoncomponents in the tubes 16 will be condensed, and the liquid nitrogenand argon components will flow downwardly through the tubes 16 andcollect in a pool 35 in the chamber 19. However, due to the relativelylow boiling point of the hydrogen, the hydrogen component of the gaseousmixture remains in gaseous phase. The downward flow of the liquidnitrogen and' argon components in the tubes 16, due to the action ofgravity, produces a suction or pumping action which draws gaseousfraction, including the incondensable hydrogen component, from thechamber 17 into the tubes 16. However, for reasons mentioned above, thepumping action of the liquid components will not cause the gaseoushydrogen component to flow downwardly in the tubes 16 and into thechamber 19.

In accordance with the principles of the present invention theaccumulation of 'incondensable hydrogen component in the tubes 16 isprevented by establishing a below the lower ends of the tubes 16 andabove the uppermost fractionating tray. The tubes 16 are housed in achamber 21 formed by a cylindrical side wall member 22, and transversepartition members 23 and '24 which separate the chamber 21 from thechambers 17 and 19. ,The annular chamber 19 is provided with a gaseouswithdrawal conduit 25 communicating with the chamber at a level belowthe lower ends of 'the small diameter tubes 16, and with liquiddistributing conduits 26 extending from the plate member 20 downwardlyinto the fractionating zone of the column. Conduits27 and 28 communicate with the chamber 21 for circulating a stream of cold fluidthrough the chamber in out-of-eontact heat exchange relation with thefluid flowing through the tubes 15 and-16. In order'to liquefy thenitrogen component of the gaseous fraction, the circulating fluid is ata temperature below the boiling point of nitrogen at the existingpressure. According to the above-mentioned Fedorko application, byoperating the column 10 at the proper pressure, a suitable fluid forthis purpose may comprise a stream of gaseous nitrogen withdrawn fromthe high pressure section of the two stage air fractionating column. Asshown,a stream of high pressure gaseous nitrogen is conducted through aconduit 29 to a boiling coil immersed in the pool of liquid argon 13.The liquid argon is at a relatively low temperature due to the columnpressure, and the high pressure nitrogen stream is cooled upon passingthrough the boiling coil while the h forced circulation of the gaseousfraction downwardly through the small diameter tubes 16 into the chamber19. This is accomplished by withdrawing a stream of gaseous mixture,including the incondensable hydrogen component, from the chamber 19through the conduit 25. This manner of flowing the gaseous fractionthrough the condenser prevents the accumulation of incondensable gasesin the chamber 17 and in the small diameter tubes 16 and assures uniformheat exchange relationship between the condensable components of thegaseous mixture and the relatively colder fluid in the chamber 21 andprovides a co'ntinous supply of reflux liquid in accordance with columnrequirements.

The liquid reflux requirement of the fractionating col umn 10 does notnecessitate liquefying the total nitrogen component or the gaseousfraction and the unliquefied po'rtion of the nitrogen component iswithdrawn through the conduit 25 together with the incondensable hydro--gren component and gaseous argon.

The withdrawn gaseous stream may comprise from 50% to nitrogen, from 8%to 12% hydrogen and the balance argon. Therate of flow of the gaseousmixture, through the conduit 25 is determined in accordance with columnoperating conditions and the condenser structure -14 is designed toprovide the required reflux production. Thus the rate of flow of thegaseous mixture withdrawn from beneath the condenser tubes 16, in thedisclosed example, is determined by column operating conditions and isin excess of the rate of flow required to establish the forced downwardcirculation through the condenser tubes 16. For the latter purpose it isonly necessary to withdraw the stream including incondensables at a ratesuflicient to establish a pressure drop along the tubes 16 for efiectingthe downward flow. Therefore, in cases in which the total condensablecomponent of the gaseous mixture is required t9 lee-liquefied liquidreflux for the column esteem or for other fractionating zones or forother purposes, only theincondensable component of the gaseous fractionor the incondensable portion and the necesary proportion of thecondensable component to establish the required pressure drop, arewithdrawn from the zone be- 'neath the condenser tubes 16.

The mere withdrawal of a relatively large volume of gaseous mixture fromthe condenser structure do'es not result in overcoming the loss ofcondensing action due to the incondensable gases. It has been determinedfrom actual experimentation with a fractionating column con structedaccording to the example described above that the withdrawal of agaseous stream including from 50% to 80% nitrogen, from 8% to 12%hydrogen and the balance argo'n from the chamber 17 through the upperendof the dome -18 does not prevent the incondensable gases fromentering the condenser tubes 16. Therefore the novel method provided bythe present invention which eliminates problems resulting from thepresence of incondensables in the gaseous fraction includes thecombination of steps for maintaining a pressure drop across thecondenser tubes 16 to establish a forced downward flow of fluid in thecondenser tubes by withdrawing a stream of gaseous mixture, includingthe incondensables, from a zone communicating with the lower ends of thecondenser tubes and isolated from the fractionating zone with respect togas.

The method provided by the present invention is applicable to thefractionation of all gaseous mixtures including incondensables in excessof one percent of the gaseous mixture in which at least a portion of thegaseous fraction of the fractionating operation is required to beliquefied. Thus the present invention has application to the separationof a wide range of hydrocarbons. for example, in addition to theseparation of a reaction mixture comprising argon, nitrogen and hydrogenas described above. Reference therefore will be had to the appendedclaims for a definition of the limits of the invention.

What is claimed is:

1. The method of producing liquid reflux for a fractionating operation,in which operation a mixture of component gases comprising from tonitrogen, from 1 to 5% hydrogen and the balance argo'n is fed to afractionating zone producing a liquidargon fraction and a gaseousfraction including nitrogen and hydrogen components, the methodcomprising the steps of conducting a stream of gaseous fraction to afirst chamber communicating with the upper ends of a plurality of spacedvertically disposed elongated heat exchange passageways, withdrawing astream of gaseous fraction from a second chamber communicating with thelower ends of the heat exchange passageways and from heat exchangerelationship with relatively cold fluid surrounding the heat exchangepassageways so that the gaseous fraction in the first chamber flowsdownwardly thro'ugh the heat exchange passageways into the secondchamber, passing a stream of cold fluid at a temperature below theboiling point of the nitrogen component at the existing pressure inout-o'f-contact heat exchange relation with the gaseous fraction flowingdownwardly in the heat exchange passageways to provide liquefiednitrogen component, the liquefied nitrogen component flowing downwardlyin the heat exchange passageways and collecting in the second chamber,and withdrawing a stream of liquefied nitrogen component from the secondchamber and introducing the withdrawn liquefied nitrogen component intothe fractionating zone as liquid reflux, the stream of gaseous fractionwithdrawn from the second chamber including the hydrogen componentcomprising from 8% to 12% of the withdrawn stream of gaseous fraction.

'2. The method defined by claim 1 in which the stream of gaseousfraction withdrawn from the second chamher includes from 50% to nitrogenand from 8% to 12% hydrogen.

'3. The method of producing liquid reflux in a fractionating operation,in which operation a mixture of component gases comprising from 10% to25% nitro gen, from 1% to 5% hydrogen and the balance argon is fed to afractionating zo'ne producing a liquid argon fraction and a gaseousfraction including nitrogen, hydrogen and argon components, the methodcomprising the steps of conducting a stream of gaseous fraction to afirst chamber communicating with the upper ends of a plurality of spacedvertically disposed elongated heat exchange passageways, withdrawing astream of gaseous fraction from a second chamber in communication withthe lower ends of the heat exchange passageways and from heat exchangerelationship with relatively cold fluid surrounding the heat exchangepassageways so' that the gaseous fraction in the first chamber'flowsdownwardly through the heat exchange passageways into the secondchamber, passing a stream of cold fluid 'at 'a temperature below theboiling point of the nitrogen com ponent at the existing pressure inout-of-conta'ct heat exchange relation with the gaseous fraction flowingdownwardly in the heat exchange passageways to provide liquefiednitrogen component, the liquefied nitrogen component flowing downwardly'in the. heat exchange passageways-and collecting in the second chamber,and withdrawing a stream of liquefied nitrogen component from the secondchamber and introducing the withdrawn stream of liquefied 'nitrogentcomponent into the fractionating zone as liquid reflux, the stream ofgaseous fraction withdrawn from the secon'd'cha'mber including thehydrogen component and comprising-from 50% to 80% nitrogen, from 8% to12%"hydrogen and the balance argon. i

4. In operation of a condenser adapted to liquefy gaseous mixtureincluding a condensable component and an incondensable componentcomprising at least one percent of the gaseous mixture, thecondenserbeing of the type including an upper chamber'to which thegaseous mixture is fed and a lower chamber connected to the upperchamber by a pluralityof vertically disposed tubes surrounded-by arelatively cold fluid at a temperature below the boiling point of thecondensable component at the existing pressure, and in which the gaseousmixture fed to the upper chamber flows downwardly into the tubes andcondensable component is liquefied in the tubes by heat interchange withthe relatively cold fluid and liquefied condensable component flowsdownwardly in the tubes to the lower chamber producing a suction eflectdrawing additional gaseous mixture from the upper chamber downwardlyinto the tubes, the method of maintaining continuous liquefaction ofcondensable component of the gaseous mixture in the tubes whichcomprises withdrawing a gaseous stream from the lower chamber and fromheat exchange relationship with the relatively cold fluid, andcontrolling the rate of flow of the gaseous stream to establish apressure drop across the tubes sufficient to prevent blocking of thetubes by accumulation of incondensable component of the gaseous mixturein the tubes, the gaseous stream including incondensable component ofthe gaseous mixture.

5. In operation of a condenser adapted to liquefy gaseous mixtureincluding a condensable component and an incondensable componentcomprising at least one percent of the gaseous mixture, the condenserbeing of the type including an upper chanmber to which the gaseousmixture is fed and a lower chamber connected to the upper chamber by aplurality of vertically disposed tubes surrounded by a relatively coldfluid at a temperature below the boiling point of the condensablecomponent at the existing pressure, and in which the gaseous mixture fedto the upper chamber flows downwardly into the tubes and condensablecomponent is liquefied in the tubes by heat interchange with therelatively cold fluid r '7 and liquefied condensable component flowsdownwardly in the tubes to the lower chamber producing a suction eilectdrawing additional gaseous mixture from the upper chamber downwardlyinto the tubes, the method of maintaining continuous liquefaction of thecondensable component of the gaseous mixture in the tubes whichcomprises withdrawing a gaseous stream from the lower chamber and fromheat exchange relationship with the relatively cold fluid, andcontrolling the rate of flow of the gaseous stream to establish apressure drop across the tubes sufiicient to prevent blocking of thetubes by accumulation of incondensable component of the gaseous mixturein the tubes, the gaseous stream including incondensable compo'nent anda portion of the condensable component.

6. In a fractionating operation in which a stream of gaseous mixture isfed to a fractionating zone producing a liquid high boiling pointfraction and a gaseous low boiling point fraction including acondensable component and an incondensable component comprising at leastone percent of the gaseous mixture, and in which gaseous low boilingfraction is fed to an upper chamber connected to a lower chamber by aplurality of vertically disposed tubes surrounded by a relatively coldfluid at a temperature below the boiling point of the condensablecomponent of the gaseous low boiling point fraction, and in which thegaseous low boiling point fraction fed to the upper chamber flowsdownwardly into the tubes and condensable component is liquefiedin thetubes by heat exchange with the relatively cold fluid and the liquefiedcondensable component flows downwardly in the tubes to the lower chamberproducing a suction efiect drawing additional gaseous low'boiling pointfraction into thetubes, the method of maintaining continuousliquefaction of condensable component of the gaseous low boiling pointfraction to provide reflux for the fractionating zone which compriseswithdrawing a gaseous stream from the lower chamber and from heatexchange relationship with the relatively cold fluid, controlling therate of flow of the withdrawn gaseous stream to establish a pressuredrop across the tubes sufiicient to prevent blocking of the tubes byaccumulation of incondensable component of the low boiling fraction inthe tubes, and withdrawing a stream of liquefied condensable componentfrom the second chamber and introducing the withdrawn stream into thefractionating zone as reflux; the withdrawn gaseous stream includingincondensable component of the gaseous mixture.

7. In a fractionating operation in which a stream of gaseousmixture isfed to a fractionating zone producing a liquid high boiling pointfraction and a gaseous 10W 'boiling'point fraction including acondensable component and an incondensable component comprising at leastone percent of the gaseous mixture, and in which gaseous low boilingfraction is fed to an upper chamber connected to a lower chamber by aplurality of vertically disposed tubes surrounded by a relatively coldfluid at a temperature below the boiling point of the condensablecomponent of the gaseous low boiling point fraction, and in which thegaseous low boiling point fraction fed to the upper chamber flowsdownwardly into the tubes and condensable component is liquefied in thetubes by heat exchange with the relatively cold fluid and liquefiedcondensable component flows downwardly in the tubes to the lower chamberproducing a suction effect drawing additional gaseous low boiling pointfraction into the tubes, the method of maintaining continuousliquefaction of condensable component of the gaseous mixture to providereflux for the fractionating zone l-which comprises withdrawing agaseous stream from the'lower chamber and from heat exchangerelationship with the relatively cold fluid, controlling the rate offlow of the withdrawn gaseous streamto establish a pressure drop acrossthe tubes sufiicient to prevent blocking of the tubes by accumulation ofincondensable component of the low boiling fraction in the tubes, andwithdrawing a stream of liquefied condensable component from the secondchamber and introducing the withdrawn stream into the fractionating zoneas reflux, the gaseous stream including incondensable component and aportion of the condensable component. 1

References Cited in the file of this patent I UNITED STATES PATENTS1,512,268

1. THE METHOD OF PRODUCING LIQUID REFLUX FOR A FRACTIONATING OPERATION,IN WHICH OPERATION A MIXTURE OF COMPONENT GASES COMPRISING FROM 10% TO25% NITROGEN, FROM 1% TO 5% HYDROGEN AND THE BALANCE ARGON IS FED TO AFRACTIONATING ZONE PRODUCING A LIQUID ARGON FRACTION AND A GASEOUSFRACTION INCLUDING NITROGEN AND HYDROGEN COMPONENTS, THE METHODCOMPRISING THE STEPS OF CONDUCTING A STREAM OF GASEOUS FRACTION TO AFIRST CHAMBER COMMUNICATING WITH THE UPPER ENDS OF A PLURALITY OF SPACEDVERTICALLY DIPOSED ELONGATED HEAT EXCHANGE PASSAGEWAYS, WITHDRAWING ASTREAM OF GASEOUS FRACTION FROM A SECOND CHAMBER COMMUNICATING WITH THELOWER ENDS OF THE HEAT EXCHANGE PASSAGEWAYS AND FROM HEAT EXCHANGERELATIONSHIP WITH RELATIVELY COLD FLUID SURROUNDING THE HEAT EXCHANGEPASSAGEWAYS SO THAT THE GASEOUS FRACTION IN THE FIRST CHAMBER FLOWSDOWNWARDLY THROUGH THE HEAT EXCHANGE PASSAGEWAYS INTO THE SECONDCHAMBER,